1. Field of the Invention
The present invention relates to an exposure method and apparatus for a picture tube and, more particularly, to an exposure method and apparatus for a picture tube in which the adhesion strength of outermost photoresist stripes to the glass panel (to be merely referred to as a panel hereinafter) of the picture tube is improved.
2. Description of the Prior Art
FIG. 1 is a schematic view of an apparatus for explaining a conventional exposure method for a picture tube. FIG. 2 shows the screen of a picture tube formed with a photoresist matrix. In order to form photoresist stripes 11 on the inner surface of the picture tube, UV light 13 is irradiated from a light source 16 to a slurry layer of photoresists 15, containing a photosensitive material and applied on an inner surface 14a of a panel 14, to fix the slurry layer. The non-fixed portion of the slurry layer is washed off in a water washing step called a "development", and a fixed portion 15a forms the photoresist stripes 11 accordingly.
Therefore, as shown in FIG. 2, the photoresist stripes 11 corresponding to slots 12a of the shadow mask 12 are formed on the inner surface 14a of the panel 14. Since the adhesion strength of the photoresists 15 and the size of the photoresist stripes 11 depend on the intensity of incident light, consideration must be given to the light intensity distribution on the entire surface of the panel 14. According to the prior art, consideration is given mostly to the distribution of incident light by means of, e.g., a filter 17 arranged between the shadow mask 12 and the exposure light source 16.
In the conventional color picture tube, both the size and pitch of the photoresist stripes 11 to be formed are large. For example, a television picture tube employs a pitch equal to or larger than 0.4 mm. If, however, the prior art is applied to a high-definition picture tube, e.g., a recent monitor tube, inconveniences occur as follows. Right and left outermost photoresist stripes 11a of the screen tend to undesirably separate, and an entirely uniform photoresist screen cannot be formed. This suggests that the adhesion strength of the photoresists 15 onto the inner surface 14a of the panel 14 degrades as a whole because the photoresist matrix formed to meet the requirement for a higher definition is small, and that some specific state occurs only near the outermost stripes. Although this special state is supposed to have occurred in the conventional picture tube as well, it did not pose a problem in a low-definition picture tube.
FIG. 3 shows in detail the optical path during exposure in order to explain the object of the present invention. Hardening of the photosensitive photoresist slurry caused by exposure and adhesion of the photosensitive photoresist slurry onto the inner surface of the panel basically depend on the quantity of the incident UV light 13, as has been described in the prior art. Light which has passed through the slurry layer of the photoresists 15 and the panel 14 is reflected inward by an outer surface 14b of the panel 14 to irradiate the inner surface 14a of the panel 14, i.e., the adhesion surface between the panel 14 and photoresists 15, to promote the adhesion effect at this portion, thereby improving the adhesion effect.
After coming incident into the photoresists 15, the light becomes diffused light directed in the direction of incidence. For the sake of simplicity, the passing light can be discussed separately as two light components, i.e., light 13b which becomes incident on the slurry layer of the photoresists 5 and is diffused, and light 13a which travels straight without being diffused. Considering the diffused light 13b, inside the periphery of the screen, light which is diffused from the peripheral portion of this inner portion overlaps the straight light 13a, and predetermined reflected light is ensured. On the outermost portion of the screen, since no light is diffused from the outside, the quantity of reflected light decreases sharply.
Regarding the light 13a which travels straight, although it is attenuated as it passes through the slurry layer of the photoresists 15, it is then refracted by the inner surface 14a of the panel 14 to become incident on the panel 14, and is reflected by the outer surface 14b of the panel 14 and is returned. The path of the return light does not necessarily coincide with the path of the incident light due to the position of the light source 16, the shapes of the shadow mask 12 and panel 14, and the positional relationship among the light source 16, the shadow mask 12, and the panel 14, but the return light lands on a point 15b which is outside an original incident point 15a. In particular, in the recent panel having a flat inner surface, the light which has passed through the outermost slot does not return along the same path.
Little light returns to the outermost photoresist stripe 11a of the panel corresponding to an outermost slot 12a1 of the shadow mask 12, and a sharp decrease in quantity of reflected light occurs at the outermost portion. This decreases the adhesion strength of the photoresist near the outermost portion, causing separation of the photoresist.